1. Field of the Invention
The present invention relates to a solid-state imaging device, and particularly relates to a solid-state imaging device that can provide high quality images even when capturing an image of a subject with high brightness, and allows large tolerances in manufacture and operational conditions.
2. Description of Related Art
In a MOS type solid-state imaging device, for reading out image signals from a pixel array in which imaging pixels are arranged in a matrix form, generally, an analog memory is provided in each column, and signals are read out from the imaging pixels on a row-by-row basis, and then, the signals kept in the column memories are outputted serially to the outside. Further, it often has been the case with a conventional example of a MOS type solid-state imaging device that a column amplifier is inserted between a pixel array and a column memory. The reason for this is that in a serial readout portion, since a high-speed operation is required therein, noise suppression hardly can be achieved, while in a column circuit portion that operates at low speed, signal amplification can be performed to provide an image signal having a high S/N ratio.
In a conventional solid-state imaging device, with respect to a signal potential V1 obtained when a floating diffusion portion (FD) of an imaging pixel is in a reset state and a signal potential V2 obtained in a state where, after the FD is reset, an electric charge generated in a photodiode (PD) is transferred to the FD, a difference between these two signal potentials is detected and amplified. This difference between the two signal potentials represents a light irradiation amount detected by the PD.
FIG. 15 shows a first example of a circuit configuration of a column amplifier in such a conventional solid-state imaging device. In this first example, first, by means of a signal from a column amplifier reset signal line 27, a column amplifier reset transistor 24 is turned on, and in this state, from an imaging pixel, a first potential V1 is inputted as a column amplifier input potential 102. A column amplifier output potential 26 obtained at this time is indicated as Vamprst. Next, in a state where the column amplifier reset transistor 24 is turned off, a signal V2 is inputted from the imaging pixel, and the column amplifier output potential 26 changes to Vamprst+(C1/C2)(V2−V1). This means that an imaging pixel signal corresponding to a light irradiation amount is amplified at a gain of C1/C2. Herein, C1 denotes a column amplifier input capacitance 21, and C2 denotes a column amplifier feedback capacitance 101. In FIG. 15, reference numeral 30 denotes a column amplifier power supply, 23 denotes a column amplifier load transistor. 22 denotes a column amplifier driving transistor, 25 denotes a gate electrode, and 31 denotes a ground (JP 05(1993)-207220 A).
Furthermore, FIG. 16 shows a second example of the circuit configuration of the column amplifier included in the conventional solid-state imaging device. This configuration differs from the configuration shown in FIG. 15 in that a column amplifier bias potential 28 is supplied to a column amplifier load transistor 23 so that a load of a grounded-source amplifier is a constant current source, and in that a clip transistor 104 is connected to an amplifier output portion. An output limit potential 103 is supplied to the clip transistor 104, thereby providing an effect of being able to avoid a phenomenon in which an amplifier output level rises to such an extent that the column amplifier load transistor 23 cannot operate in a saturation region, which hampers a constant current operation. As in the case of the conventional solid-state imaging device, with respect to a pixel signal corresponding to a light irradiation amount that is a difference between two signals V1 and V2 from an imaging pixel, this column amplifier also has the function of amplifying the pixel signal at a gain of C1/C2 that is a ratio between a column amplifier input capacitance 21 and a column amplifier feedback capacitance 101 (JP 2005-252529 A).
Now, the following discusses peculiarities of a power supply layout pattern of a MOS type solid-state imaging device in which an amplifier is provided in each column. FIG. 17 shows a typical layout configuration. A MOS type solid-state imaging device in which an amplifier is provided in each column has the following configuration. That is, a column amplifier 109 is disposed in a one-to-one correspondence with a vertical signal line 108 in each column of a pixel array 1 in which a plurality of imaging pixels 20 are arranged in a matrix form, and in order to transmit image signals to a signal output portion 110, a column amplifier-arranged portion 3 inevitably has a configuration in which a large number of the column amplifiers 109 are arranged laterally. With respect to each of the large number of the column amplifiers 109 thus arranged, amplifier power is supplied from an amplifier power supply pad 105, and a ground potential is supplied through a ground pad 106. This requires that wiring for these column amplifiers 109 be extended from each of the pads for a distance in a lateral direction, such that the degree of the influence of a parasitic resistance 107 may become too large to be negligible.
Furthermore, in the column amplifier in the conventional solid-state imaging device described above as the first example, it is inevitable that an operation current of the column amplifier changes considerably in accordance with an input signal level. Therefore, for example, considering the case of capturing an image of a subject whose brightness is high only in a central portion thereof, in a column amplifier corresponding to the central portion, when a signal V2 from a pixel is inputted, an input potential of the column amplifier becomes high to reduce an operation current, so that an applied voltage to each of the column amplifiers positioned in a peripheral portion of the column amplifier deviates from a desired voltage value. Then, in each of these column amplifiers in the peripheral portion, a difference in applied voltage may occur between when a reset signal V1 is inputted and when an optical signal V2 is inputted, resulting in the occurrence of a deviation in amplifier output to cause a deviation in black level between the right and left sides of the subject with high brightness, which has been disadvantageous.
Furthermore, in the column amplifier in the conventional solid-state imaging device described above as the second example, by the function of a clip circuit, the transistor that functions as the constant current source operates only in a saturation region, thereby allowing a variation in operation current of the amplifier to be reduced. However, other problems may occur such as an operation range of the column amplifier being narrowed due to a variation in threshold value of the clip transistor, and characteristics changing when an output of the column amplifier approaches a clip level, thus making it difficult to secure a sufficient manufacture tolerance. Moreover, it also has been disadvantageous in that photoelectric conversion characteristics also may change as a result of a variation in threshold value due to a variation in ambient temperature.